This invention relates to a heat-sensitive color developable composition. More particularly, it relates to a heat-sensitive color developable composition containing epoxy group-modified polyvinyl alcohol.
A heat-sensitive recording material is usually comprised of a heat-sensitive color developable layer formed on a support or substrate, such as a paper sheet, a plastic film or a metallized paper sheet, and adapted for recording a color developed layer therein by heating. Since recording can be made only by heating, the material is widely used not only for duplicating books or documents but also for computer output or telefax recording and for forming inscriptions on identification certificates, commutation tickets or passenger tickets. Above all, a heat-sensitive recording material mainly composed of an acidic material such as phenol compounds and usually colorless to pale-colored leuco-dyes on a support or substrate is useful because of clear color tone of produced image.
In a heat-sensitive recording material in general, a binder plays an important role in addition to color developing components. When producing the heat-sensitive recording material, leuco-dyes which are color developing agents and a developer, are separately dispersed and pulverized by dispersing devices such as ball mills or sand grinders in separate aqueous media containing water-soluble resins as the binder. The resulting dispersed liquids are mixed together, applied to a support or substrate and dried.
As the binder, water-soluble polymers, such as polyvinyl alcohols, starches, modified materials and derivatives thereof, methyl cellulose, carboxymethyl cellulose, gum arabic, gelatine, polyvinyl pyrrolidone, polyacrylamide, polyacrylate, styrene-maleic anhydride copolymer or isobutyrene/maleic anhydride copolymer, or a high polymer emulsion or latex such as polyvinyl acetate or styrene/butadiene copolymer, are so far known and used.
Above all, polyvinyl alcohols and starches, especially oxidized or etherificated starches, are preferred as the binder, since the properties thereof, such as dispersibility, film strength, color developability, sticking, non-dust adhesion and non-cohesiveness, required as the binder, are more excellent than those of the other materials. However, the properties obtained so far are not fully satisfactory since prolonged heating or recording results in adhesion of dusts to a thermal head due to abrasion of the thermal head or sticking of the head to the heat-sensitive recording paper under conditions of elevated temperatures. In addition, problems are presented such that, because of insufficient water resistance, inadvertent erasure of printed characters or figures, finger stickiness of the film surface or peeling may be caused on contact of the printed recording paper sheet with water or a plasticizer and the image density may be lowered after recording, or the printed image may become completely illegible.
As the water resistant agents for polyvinyl alcohols or starches, it is known to use melamine-formaldehyde condensates, epoxy compounds, glyoxal, chromium alum or glutaraldehyde. However, these water resistant agents are inconvenient in that the coated liquid may cohere or lower in color developability and heat treatment under elevated temperatures is required for obtaining a sufficient water resistance. Above all, heat treatment under elevated temperatures is not desirable since the problem of color development may be presented in the production of the heat-sensitive recording material.